Earth's atmosphere hasn't had this much CO2 in human history, and possibly not since the Pliocene Epoch about 3 million years ago.

Earth's atmosphere is changing more quickly than ever before in human history, and it's no secret why. Humans are releasing a flood of greenhouse gases, namely carbon dioxide, into the air by burning fossil fuels. CO2 lingers in the sky for centuries, so once we reach a certain level, we're stuck for a while.

Until recently, our air hadn't contained 400 parts per million of CO2 since long before the dawn of Homo sapiens. It briefly broke 400 ppm in the Arctic in June 2012, but CO2 levels fluctuate with the seasons (due to plant growth), so they soon dipped back into the 390s. Hawaii then saw 400 ppm in May 2013, and again in March 2014. The Mauna Loa Observatory also averaged 400 ppm for all of April 2014.

And now, in yet another miserable milestone, the entire planet has averaged above 400 ppm for a full month. That's according to the U.S. National Oceanic and Atmospheric Administration (NOAA), which reported this week that Earth's average CO2 level was 400.83 ppm during March 2015.

"Reaching 400 parts per million as a global average is a significant milestone," says Pieter Tans, lead scientist of NOAA's Global Greenhouse Gas Reference Network. "This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times. Half of that rise has occurred since 1980."

To put this in perspective, we know (thanks to ice-core samples) that CO2 levels hadn't even flirted with 400 ppm for at least 800,000 years before this century. The history is hazier beyond that, but research suggests CO2 levels haven't been this high since the Pliocene Epoch, which ended about 3 million years ago. Our own species, by comparison, only evolved about 200,000 years ago.

The peaks and valleys in this graph show seasonal variability, but the long-term trend is clear. (Image: NASA)

"Scientists have come to regard [the Pliocene] as the most recent period in history when the atmosphere's heat-trapping ability was as it is now," explains the Scripps Institution of Oceanography, "and thus as our guide for things to come." (For anyone who isn't aware, CO2 traps solar heat on Earth. There's a long historical link between CO2 and temperature; see more about that here.)

So what was the Pliocene like? Here are some key features, per NASA and Scripps:

Sea level was about 5 to 40 meters (16 to 131 feet) higher than today.

The poles were even hotter — as much as 10 degrees C (18 degrees F) more than today.

CO2 is a key part of life on Earth, of course, and lots of wildlife flourished during the Pliocene. Fossils suggest forests grew on Ellesmere Island in the Canadian Arctic, for example, and savannas spread across what is now North African desert. The problem is that we've built up swaths of fragile human infrastructure in just a few generations, and the abrupt return of a warmer, wetter Pliocene-esque atmosphere is already starting to wreak havoc with civilization.

Extreme weather swings can lead to crop failures and famines, for example, and rising sea levels endanger about 200 million people who live along the planet's coastlines. The Pliocene was prone to "frequent, intense El Niño cycles," according to Scripps, and lacked the significant ocean upwelling that currently supports fisheries along the west coasts of the Americas. Corals also suffered a major extinction at the Pliocene's peak, and an encore of that could threaten an estimated 30 million people worldwide who now rely on coral ecosystems for food and income.

While the Pliocene might be a useful guide, there is a key difference: The Pliocene climate developed slowly over time, and we're reviving it at an unprecedented pace. Species can usually adapt to slow environmental changes, but even we might have trouble keeping up with our own upheaval.

"I think it is likely that all these ecosystem changes could recur, even though the time scales for the Pliocene warmth are different than the present," Scripps geologist Richard Norris said in 2013. "The main lagging indicator is likely to be sea level just because it takes a long time to heat the ocean and a long time to melt ice. But our dumping of heat and CO2 into the ocean is like making investments in a pollution 'bank,' since we can put heat and CO2 in the ocean, but we will only extract the results over the next several thousand years. And we cannot easily withdraw either the heat or the CO2 from the ocean if we actually get our act together and try to limit industrial pollution — the ocean keeps what we put in it."

An artist's rendering of Pliocene wildlife, including a brave beaver standing up to a saber-toothed cat. (Image: U.S. National Park Service)

There's nothing magical about 400 molecules of CO2 in every 1 million molecules of air — their greenhouse effect is about the same as 399 or 401 ppm. But 400 is a round number, and round numbers are natural milestones, whether it's a 50th birthday, a 500th home run or the 100,000th mile on an odometer. And when it comes to CO2, even a symbolic milestone is important if it can draw more attention to how quickly and dramatically we're changing our planet.

"This milestone is a wake-up call that our actions in response to climate change need to match the persistent rise in CO2," says Erika Podest, a carbon and water-cycle expert at NASA's Jet Propulsion Laboratory. "Climate change is a threat to life on Earth and we can no longer afford to be spectators."